1. Field of the Invention
The present invention relates in general to a left ventricular assist device, and in particular to an apparatus and method for supporting the blood circulation when the heart is severely injured and is unable to maintain a systemic arterial pressure adequate to support the inside walls of patient's aorta. Still more particularly, the present invention relates to an apparatus and method for supporting and expanding the walls of the aorta from collapse during operation of the device, which might otherwise occur due to extremely low blood pressure and for providing diastole/systole-like cardiac function in a patient with a severely diseased or injured heart.
2. Description of the Prior Art
In the United States alone, 60,340,000 people have cardiovascular disease. Of these, over 2,000,000 have congestive heart failure, with more than 500,000 new cases diagnosed each year. In 1995, only 2,359 patients received heart transplants, the most permanent of treatments to date, while 770 patients who qualified for heart transplant died waiting. About 450,000 patients undergo open-heart surgery each year, and 2% of these cases require mechanical cardiac support after surgery at a cost of about $400,000 per survivor.
There are many different causes of heart failure, the most common of which are (1) acute myocardial infarction; (2) cardiomyopathy; (3) cardiac valvular dysfunction; (4) extensive cardiac surgery; and (5) uncontrolled cardiac arrhythmias. Heart failure, especially resulting from disease or damage to the left ventricle of the heart, can result in many problems.
Problems arising from left ventricle disfunction are particularly troublesome since the heart itself, as well as the rest of the body, depends on left ventricle function for oxygenated blood to maintain aerobic respiration. Heart failure results in ineffective emptying of the failing left ventricle during systole, which can result in (1) decreased cardiac output; and (2) elevated left atrial and pulmonary venous pressures, which cause pulmonary congestion and edema. Pulmonary congestion and edema, or tissue swelling, prevent effective oxygenation of the arterial blood, and coupled with reduced cardiac output, and can lead to tissue hypoxia. In the final stage of congestive heart failure, a vicious cycle leads to progressive systemic hypotension (low blood pressure), hypoxemia (oxygen depletion), tissue anoxia, further depression of cardiac function and cardiac arrhythmias, and ultimately, death.
Left ventricular assist devices (LVAD) are an alternative treatment for heart failure to transplantation or medical therapy. Patrick M. McCarthy et al., Cardiopulmonary Support and Physiology, 115 J. Thoracic Cardiovascular Surgery 904 (1998). LVADs that are presently available in clinical settings or being developed are of several types: (1) the heart-lung machine, providing cardiopulmonary bypass; (2) intra-aortic balloon pumps (IABP), which reduce resistance to left ventricular ejection and augment diastolic pressure; (3) pumps positioned in parallel with the left ventricle; and (4) complete artificial hearts. Of these, only the IABP devices can be implemented without surgically opening the chest and operating on the heart and/or major blood vessels. Further, after improvement of left ventricular function, only the IABP devices can be removed without opening the chest and operating on heart and blood vessels. Thus, this device has become, by far, the most commonly utilized LVAD. However, the application of present IABP devices is limited to less severecases, since the IABP can function effectively only if the left ventricle is able to eject an adequate output to maintain a mean systemic arterial pressure greater than 60 mmHg. This greatly limits the practical use of present IABPs.
When the heart is injured to the point that mean systemic arterial pressure is less than 60 mmHg, the aorta will collapse during the deflation phase of present IABPs. There are no prior art devices that can be used to support the walls of the aorta so that the IABP can effectively assist heart function. Some stent devices have been used to mechanically support smaller arteries, especially the coronary arteries. But these devices are permanently placed within smaller arteries, and are not designed to be used in conjunction with other devices placed within the artery that assist in increasing systemic arterial blood pressure. There is yet a need for a device that can be used to support the aorta, can be used in conjunction with IAPBs, and is removable. With such a device, blood can be sucked from the defective left ventricle and then pumped to the systemic circulation, essentially replacing the function of a severely injured heart.
Therefore, the present invention is directed towards a LVAD device that can act as a temporary replacement for the function of the left ventricle in a patient whose heart is severely diseased or injured and is unable to maintain a systemic arterial pressure adequate to support the aorta, as required for effective operation of present IABPs.